(1) The particle radii of the 3 plant viruses, turnip crinkle virus (TCV), hibiscus chlorotic ringspot virus (HCRSV) and pelargonium flowerbreak virus (PFBV) were determined by agarose gel electrophoresis in the presence and absence of 5 mM calcium as 12.8, 20.3, 18.3 and 29.4, 24.2 and 22.1 nm respectively. This size determination uses the entire nonlinear Ferguson plot in conjunction with computer simulation, and commercially available polystyrene microsphere size standards in the size range 20 to 60 nm. (2) Ferguson plots of bacteriophages, plant viruses and polystyrene sulfate particles with two or more points of inflection are interpreted in terms of a varition of particle size (compressibility) as a function of gel concentration. This particle size effect is superimposed on the variation of gel fiber dimension in causing the convex-sigmoidal curvature. These plots also serve as characteristic "finger prints" for the particle. (3) Ferguson plots of plant viruses from crude extracts are presently attempted in order to demonstrate pattern identity with purified virus, absence of perturbation by subcellular particles or nucleic acids, and readiness to attack clinical virus separation and identification problems.